ARTICLE pubs.acs.org/EF
Evaluation of Structural Characteristics of Huadian Oil Shale Kerogen Using Direct Techniques (Solid-State 13C NMR, XPS, FT-IR, and XRD) Jianhui Tong,†,‡ Xiangxin Han,† Sha Wang,† and Xiumin Jiang*,† †
Institute of Thermal Energy Engineering, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, P R China ‡ School of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen, 333001, P R China ABSTRACT: A sample of kerogen isolated from Huadian oil shale was studied using a combination of solid-state 13C NMR, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR), and X-ray diffraction (XRD) techniques to evaluate its structural characteristics. 13C NMR results indicate that the carbon skeletal structure of this kerogen is mainly composed of a fairly high fraction of aliphatic carbon (86.1%), with a very low aromaticity (fa) of 9.7%; methylene (CH2) carbons dominate in all types of aliphatic carbons and the majority of them exist as many long straight chains but not saturated alicyclics. The average methylene carbon chain length (Cn) is between 12 and 24. There are only one fused aromatic ring (e.g., naphthalene) or two single aromatic rings (one benzene ring and one penta-heterocycle) per 100 carbon atoms. However, aromatic rings in this kerogen have a very high value of substitutive degree (δ = 0.42 0.75). Furthermore, XRD analysis suggests that most methylene straight chains and aromatic carbons can not form crystalline but amorphous structure and are linked to each other by various bridge bonds and methylene (CH2) chains. FT-IR, XPS, and 13C NMR results show that organic oxygen in the kerogen exists as mainly three types of oxygen functional groups. Both XPS and 13C NMR results agree on the same ordering of their respective contents: C O and C OH groups are dominant, followed by OdC O, and CdO or O C O groups. The 13C NMR results further suggest that more oxygen of C O and C OH groups is bound to aromatic carbons. XPS shows that over half the total amount of organic nitrogen in Huadian kerogen exists as aromatic heterocycles, which concludes pyrrolic nitrogen richest in total organic nitrogen, pyridinic, and protonated-pyridinic forms. A relatively high content of amino nitrogen over 30 mol % also is present in this kerogen, which is much higher than that of other same type kerogens. Organic sulfur is distributed in this kerogen as aromatic and aliphatic sulfur, sulfone, and sulfoxide in the order of the relative mole fraction.
1. INTRODUCTION Oil shale has been used for retorting to yield shale oil and burning directly as a fuel to generate electricity or heat for many years. However, further developing the two primary industrial technologies for the utilization of oil shale is limited due to economic and environmental factors. In view of this, considering fully oil refining, generating electricity, utilization of ash, economic effectiveness, and environmental protection, etc., Jiang et al.1 put forward the “New technology for the comprehensive utilization of Chinese oil shale resources”. Retorting oil shale to yield oil is an important part of the new technology, which is evaluated by the yield and quality of shale oil as two key indexes. Many studies have demonstrated that the yield and quality of shale oil mainly depend on two main factors: the external one includes particle grain size2 of oil shale and various processing parameters3 7 of retorting, such as pyrolysis temperature, heating rate, and pyrolysis atmosphere, etc.; the inherent one is the chemical structure and composition of oil shale kerogen. As for the latter, Miknis et al. have found that there is a strong correlation between total aliphatic carbon and Fischer assay oil yield by solid-state 13C nuclear magnetic resonance (NMR) or Fourier transform infrared (FT-IR) methods,8 13 while aromatic carbon gives little contribution to oil yield.14 Although the optimal design for retorting system and processing parameters r 2011 American Chemical Society
can help to improve the yield and quality of shale oil, this is constrained by the intrinsic factor eventually. Therefore, for seeking the better measurement for further improving the technical level of retorting to yield shale oil, more likely, it has a practical significance for investigating the chemical structure of oil shale kerogen and understanding its mechanism of thermochemical conversion microscopically. Despite that organic oxygen, nitrogen, and sulfur heteroatoms are present in low amounts relative to organic carbon and hydrogen in kerogen, they are associated with carbon atoms as various structural forms, which are also the key parts of the whole molecular structure of kerogen. These functional groups involving heteroatoms can be changed into various compounds and distributed in gases, shale oil, shale char, and ash during oil shale retorting and combustion, making a disadvantageous effect on both environment and quality of shale oil products.15 21 Therefore, for understanding the chemical structure of kerogen more comprehensively, it is necessary to investigate the heteroatom structural forms and their distributions, respectively; more important, this also contributes to the study work on the forming Received: May 17, 2011 Revised: August 2, 2011 Published: August 02, 2011 4006
dx.doi.org/10.1021/ef200738p | Energy Fuels 2011, 25, 4006–4013
Energy & Fuels
ARTICLE
H
10.54
Oa
8.08
N
1.72
Stb
2.14
H/Cc
1.63
O/Cc
0.078
the structures of carbons present in this kerogen, with FT-IR and XRD as two supplementary measurements. FT-IR is used to obtain some qualitative information about organic oxygen functional groups; both solid-state 13C NMR and XPS are employed to quantify organic oxygen forms. Nitrogen and sulfur species are characterized by XPS. The purpose of this study is to obtain understanding of the chemical structural features of Huadian kerogen by these direct measurements. Moreover, this study is also necessary or f further work—building a macromolecular model for this kerogen for investigating its pyrolysis mechanism.
N/Cc St/Cc
0.019 0.01
2. EXPERIMENTAL SECTION
Table 1. Elemental Analysis of Huadian Oil Shale Kerogen (Wt %, Dry Ash Free Basis) C
a
77.52
By difference. b Total sulfur. c Atomic ratio.
mechanism of these products for finding ways to reduce pollutants. There are many techniques for studying kerogen structure, which are divided into two major categories: direct (physical) and indirect (chemical) methods. Compared with the latter, the outstanding advantage of the direct one is that solid samples are analyzed as a whole and without alteration; as a result, the structural information destroyed by decompositional methods is preserved. On the other hand, with the further improvement of direct solid state measuring techniques (including 13C NMR, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and FT-IR, etc.), they can offer more and more accurate information about kerogen structure. Hence, these techniques have been employed by more and more investigators to study oil shale kerogens. Yen et al. studied, for the first time, asphaltenes22 and Green River oil shale kerogen23 by using the XRD method, this work is very helpful to both elucidate the structure and build macromolecular models of these complex organic materials. However, the amount of information provided by this method is not more abundant than by solid-state 13C NMR. In recent years, solid-state 13C NMR has been used for investigating the structure of oil shale kerogens. Hagaman et al.24 considered that the aliphatic carbon consists of condensed alicyclics and normal long-chain hydrocarbons in terms of the shape of aliphatic resonance bands of a Green River kerogen. Subsequently, Trewhella et al.25 estimated a Green River kerogen structure from carbon functionality data obtained by 13C NMR. In addition, 13 C NMR investigations have also been made on Estonian kukersite26 and Aleksinac27 kerogens. Although the three kerogens derived from different countries are classified into the same type I kerogen, there is some difference in structure among them according to each carbon functionality data obtained. This suggests the complexity and variety of structure of kerogens. As to the direct methods for studying the structures of organic heteroatoms, FT-IR is one common method used for analysis of organic oxygen functional groups in complex organic solids.28 31 Although some information about oxygen functional groups can be obtained from this technique, the results of quantitative analysis offered by it are not reliable enough.28,29 XPS and solid-state 13C NMR techniques can overcome this deficiency and quantify organic oxygen species, which have been proven in the literature.32 35 Not only that, XPS also is a good tool for directly investigating organic nitrogen35 37 and sulfur species35,38 in complex carbonaceous materials such as coal and kerogen. In this paper, several direct techniques are used to characterize the chemical structure of Huadian kerogen, China. Solid-state 13 C NMR is used for the qualitative and quantitative analysis of
2.1. Samples. Oil shale was obtained from the fourth seam of Dachengzi deposit located in Huadian city, China. To obtain the kerogen concentrate for analysis, the shale sample was crushed and ground to fine powders (
